The invention pertains to an absorbent, porous structure, intended for use in an absorbent article, wherein the structure exhibits a first region, primarily consisting of a first material, which stands in direct connection with a second region, primarily consisting of a second material.
A large problem, in connection with the construction of absorbent bodies for absorbent articles of the discussed type, is to achieve an optimum combination of a sufficiently large liquid acquisition ability, sufficient local and total absorption capacity and sufficient liquid distribution ability. Furthermore, it is essential that the absorbent article is able to retain absorbed body fluid so that rewetting, i.e. liquid passage back out from the article, is avoided. Another important property of, above all, diapers and incontinence protectors is that the article repeatedly should be able to receive and absorb relatively large liquid quantities, emitted during a short period of time.
One type of commonly occurring absorbent bodies for absorbent articles consists of one or several layers of cellulose fluff pulp. When such an absorbent body is wetted, the region of the absorbent body which initially is hit by the liquid will absorb essentially all liquid. Thereby, this region is saturated with liquid and, when subsequent wettings occur, the absorbent body does not have sufficient capacity in order to receive all excreted body fluid. Accordingly, the liquid will flow out over the surface of the article and leak out over the edges of the article.
In order to remedy such leakage, it has been suggested to provide the absorbent body with compression patterns of different types and thereby to increase the liquid distribution ability of the article. One example of such a compression pattern is grooves which extend in the longitudinal direction of the article. In this way, it is possible to achieve a certain drainage of the initially wetted region on the absorbent body, since the finer capillaries in the compressed portions of the absorbent body transport liquid better than the surrounding portions of the absorbent body. Such capillary transport, however, takes place slowly and the draining of the wetted region of liquid will therefore often be incomplete and insufficient.
Another problem, in connection with compressed fibre structures, is that the compressed regions swell when wetted, whereas surrounding, less compressed regions often collapse. Thereby, the initial differences in capillary size in the structure are levelled out, and the liquid distribution ability of the fibre structure is impaired.
One way of avoiding that liquid flows out onto the surface of the absorbent body is to arrange two or more absorbent layers with mutually different properties on top of each other. For example in accordance with WO 93/15702, an upper layer, intended to be facing the user during use, may thereby consist of cellulose fluff pulp with a high critical bulk and a comparatively coarse capillary network, while a lower layer consists of a layer of cellulose fluff pulp with lower critical bulk and finer capillaries. Thereby, critical bulk refers to the bulk at which the fluff pulp neither swells nor collapses when wetted.
The intention with such a construction is that the liquid rapidly should be allowed into the upper, more porous layer, and then gradually be emptied of liquid by means of the upper layer being drained by the finer capillaries in the lower layer. The expectation is that the upper layer should be sufficiently emptied of liquid in order to avoid leakage when the absorbent body once again is hit by body fluid. However, it has been found that in practice this will not work as expected. The reason for this is that the surface properties of the fibres in the two cellulose fluff layers are such that liquid drainage from the upper layer to the lower layer does not take place to the extent which might be expected from the difference in capillary size.
One type of cellulose fluff pulp with high critical bulk is chemi-thermomechanically manufactured fluff pulp, so-called CTMP. In a structure of the type which is disclosed in WO 93/15702, CTMP is combined with chemically manufactured fluff pulp, so-called CP, which has a lower critical bulk. Such pulps initially also exhibit a difference in hydrophilicity, or wettability, wherein CTMP is less hydrophillic than CP. Such a difference in hydrophilicity facilitates the liquid transport in a direction from a region consisting of CTMP to a region consisting of CP.
During wetting, however, the surface properties of the cellulose fibres change, so that also cellulose fluff pulp which in a dry state exhibits low wettability instead becomes more hydrophillic. The reason for this, amongst other things, is that the surface-chemical properties of the pulp fibres are changed because a reorientation occurs at the fibre surfaces so that hydrophillic groups are concentrated, as a result of which the fibre surfaces become more wettable. Another reason, contributing to the changed surface properties, is that a change takes place also with regard to resins and other components, for example by means of certain components dissolving, while other, more hydrophillic components migrate towards the fibre surfaces.
By means of the present invention, however, an absorbent structure of the type mentioned in the introduction has been achieved in which the problems with liquid transfer between the regions in the absorbent body, consisting of absorption materials with different surface properties, have been essentially eliminated.
An absorbent structure according to the invention is primarily characterized in that the receding wetting angle xcex8r, is larger for the first material than for the second material, as a result of which liquid transport between the two regions takes place in a direction from the first region to the second region when the porous structure is wet.
According to one advantageous embodiment, also the advancing wetting angle xcex8a is larger for the first material than for the second material, as a result of which liquid transport takes place from the first region to the second region, irrespectively of whether the structure is dry or wet.
It is advantageous for the liquid transfer between the regions in the absorbent structure if the average pore size in the absorbent structure is larger within the first region with the first material than within the second region with the second material. Since, for example, a porous fibre material exhibits pores, or voids with different sizes within a size interval, it is impossible to define an exact pore size. The expression xe2x80x9caverage pore sizexe2x80x9d refers to an average of the size of the pores in the absorbent structure. Thereby, it is desirable that the majority of the pores have a size which is close to the average pore size. This implies that the pore size variation is small and that the liquid transportation properties of the structure are easier to predict, based on the knowledge of the average pore size.
According to one embodiment of the invention, the first region is constituted of a first layer in the absorbent structure, and the second region is constituted of a second layer in the absorbent structure, wherein the two layers stand in direct connection with each other via surfaces of the layers bearing on each other.
Alternatively, the two regions can be constituted of parts of the one and the same material layer. Thereby, the division of the material layer into different regions may be such that the first region and the second region are arranged next to each other in the plane of the material layer. However, it is also possible to design an absorbent structure according to the invention exhibiting a material layer where the two regions of material with different surface properties are arranged next to each other in the thickness direction of the material layer.
Still another possibility, within the scope of the, invention is to arrange a number of regions with mutually different receding wetting angles, so that a wetting angle gradient is formed in the absorbent structure. Such a wetting angle gradient may occur in a substantially planar absorbent structure, exhibiting a thickness direction and two opposing main surfaces. Thereby, a wetting angle gradient in the thickness direction of the structure may be achieved by means of the structure being built up from a number of layers, wherein the receding wetting angle decreases in a direction from one surface of the structure towards the other surface.
In a corresponding way, a wetting angle gradient may be created in the plane of the structure by means of arranging regions with different receding wetting angles next to each other in the plane. It is of course also possible within the scope of the invention to conceive a structure exhibiting a wetting angle gradient both in the thickness direction and in the plane.
It is further an advantage if the regions in the structure also exhibit a gradient in the advancing wetting angle.
According to another embodiment of the invention, the first material in itself exhibits a receding wetting angle which is essentially as large as, or smaller than the receding wetting angle of the second material. In order to achieve the desired difference in wetting angles between the two material regions in the absorbent structure the first material is treated with an agent in order to raise the receding wetting angle above the value of the receding wetting angle of the second material.
The invention has been found to be well suited in connection with absorbent structures wherein the first region primarily consists of chemi-thermomechanical cellulose fluff pulp (CTMP), and the second region consists of chemical cellulose fluff pulp (CP), and wherein the surface of the CTMP fibres has been treated with an agent in order to increase the receding wetting angle. With such a treatment of CTMP fibres it has been found to be possible to raise the receding wetting angle from between 0xc2x0-10xc2x0 to approximately 40xc2x0, something which gives the absorbent structure considerably improved liquid transportation properties by means of favourably influencing the liquid transfer between adjacent regions with different fibre structure.
Other absorption materials which may be used when designing an absorbent structure according to the invention are different types of absorbent foams, absorbent, bonded or unbonded fibre structures completely or partly consisting of absorbent fibres such as cotton, viscose, peat moss, flax, or the like.
A useful agent for raising the receding wetting angle is ethyl-hydroxy-ethylcellulose (EHEC) which is applied in the absorbent structure, for example by spraying or coating with a liquid containing the agent, for example in the form of a solution or suspension, or by any other known method for surface treatment.
The absorbent structure according to the invention may further constitute all or a part of an absorbent body in an absorbent article, such as a diaper, a sanitary napkin, or an incontinence protector. Such an absorbent article exhibits a liquid-pervious cover layer, a liquid-impervious cover layer, an absorbent body enclosed between the two cover layers. In a case where the first region is constituted of a first layer in the absorbent body and the second region is constituted of a second layer in the absorbent body, the first layer is suitably facing the liquid-pervious cover layer and the second layer is facing the liquid-impervious cover layer.
As a rule, an absorbent article has an elongate shape with two end portions and a crotch portion, arranged between the end portions, intended to be arranged in the crotch of a user during the use of the article and thereby to serve as a reception region for the body fluid which is emitted to the article. Thereby, it is advantageous that the first region, consisting of the first material, substantially coincides with the crotch portion of the article.
By means of ensuring that the material regions in the absorbent structure exhibit differences in wetting angles at least in a wet state, but preferably also in a dry state, it is possible to obtain a controlled and predictable liquid distribution in the absorbent structure.
When a liquid droplet is placed on an even surface in a solid state, one out of two possible events will take place depending on the properties of the liquid and the solid material, respectively. The liquid may either be spread out over the surface, or remain as a droplet on the solid material. In the latter case, the droplet will form a defined angle with the surface of the solid material.
Theoretically, the contact angle xcex8 may adopt values between 0xc2x0 and 180xc2x0. In practice, however, the contact angle will never become 180xc2x0 since the force of gravity disturbs the shape of the droplet. A contact angle xcex8=0xc2x0 will imply that the liquid spreads spontaneously on the surface. The contact angle xcex8=90xc2x0 constitutes the limit for wetting. When the contact angle is smaller than 90xc2x0, liquid will spontaneously be absorbed into the pores of the material, while a contact angle above 90xc2x0 implies that a pressure must be applied in order to make the liquid penetrate into the pores. However, the limit of 90xc2x0 is true only for capillaries having parallel walls.
Dynamic contact angle refers to the angle which is exhibited when a liquid front is moving. The terms advancing and receding wetting angle are intended to specify if the dynamic contact angle is measured when a liquid advances across a dry surface, or when the liquid recedes across a recently wetted area.
The significance of the receding wetting angle, to the achievement of good liquid transfer between two components in an absorbent structure, has not previously been known. By providing a sufficiently large difference between the receding wetting angles of adjacent regions in an absorbent structure it is, accordingly, by means of the invention possible to construct an absorbent fibre structure wherein the liquid transfer properties do not change when the fibre structure is wetted. As a result of the difference in receding wetting angle, between the different regions in the fibre structure, the region which has the smallest wetting angle further has the ability to drain liquid from a region in the fibre structure with a higher receding wetting angle.
The liquid transfer between two adjacent regions in a fibre structure also depends on the pore size in the two regions. If the regions have the same pore size, the difference in receding wetting angle must be larger the smaller the pores are. This also implies that a very strongly compressed structure with small pores, in certain cases may drain a more hydrophillic, less compressed structure. In practice, however, it has been found that differences in pore size are levelled out after wetting. The reason for this is that wet structures swell or collapse so that two fibre structures which initially have different pores sizes become rather similar after wetting.